Paper feeder and image forming apparatus

ABSTRACT

When a recording medium is not detected even after an elapse of a predetermined period of time since a paper feeding unit fed the recording medium, the paper feeding unit is operated, to execute an assisting operation for assisting conveying of the recording medium.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a paper feeder for feeding andconveying a recording medium on which an image is to be formed, and animage forming apparatus including the paper feeder.

2. Description of the Related Art

In an image forming apparatus such as a printer or a copying machine,when a printing operation is started, an operation for feeding paperserving as a recording medium set in a paper feeding unit is firstexecuted. If the paper set in the paper feeding unit is recycled paper,defective paper feeding is more likely to occur as compared with highquality paper (plain paper) because the recycled paper has worseconveyance properties. When a paper feeding roller for performing apaper feeding operation is worn-out and is coming to the end of itslife, defective paper feeding is also likely to occur. The defectivepaper feeding means that paper is not correctly fed even if a conveyanceroller performs an operation for feeding the paper.

A technique for improving such defective paper feeding is proposed inJapanese Patent Application Laid-Open No. 11-334935, for example. InJapanese Patent Application Laid-Open No. 11-334935, when defectivepaper feeding occurs, a retrying operation for driving a paper feedingroller again is executed to recover the defective paper feeding, tosignificantly reduce the number of times of occurrence of a paper jam(also referred to as jam) caused by the defective paper feeding.

The above-mentioned recycled paper not only has worse conveyanceproperties but also more easily creates paper dust than the high qualitypaper. When the paper contacts the paper feeding roller and a conveyancepath when conveyed, the paper wears out, so that fine paper dust iscreated. When the conveyance roller further conveys the paper fed by thepaper feeding roller from the paper feeding unit, a conveyance force ofthe paper is reduced if the paper dust adheres to the conveyance roller.Immediately after the paper is fed, back tension is applied due to thefriction of a separation unit (e.g., a separating pad) for separating aplurality of papers set in the paper feeding roller or the paper feedingunit. Therefore, the conveyance force of the paper by the conveyanceroller is further reduced, so that the paper cannot be conveyed and maybe jammed.

In order to improve the conveyance force, a conveyance roller can beadded, then pressure for nipping paper in a conveyance roller isincreased. If the conveyance roller is added, however, the costincreases. If the pressure for nipping the paper is increased, the paperis liable to form wrinkles although the conveyance force is improved.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, an apparatus includes afeeding unit configured to execute an operation for feeding a recordingmedium and then, stop the operation, a conveyance unit configured toconvey the fed recording medium, and a detection unit configured todetect the conveyed recording medium, in which when the detection unitdoes not detect the recording medium at a predetermined timing, thefeeding unit executes an assisting operation for assisting conveying ofthe recording medium.

Further features and aspects of the present invention will becomeapparent from the following detailed description of exemplaryembodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate exemplary embodiments, features,and aspects of the invention and, together with the description, serveto explain the principles of the invention.

FIG. 1 is a schematic view of a laser beam printer according to anexemplary embodiment.

FIG. 2 is a block diagram illustrating the configuration of a controlsystem for controlling each unit in the laser beam printer illustratedin FIG. 1.

FIG. 3 is a block diagram within a paper feeding control unitillustrated in FIG. 2.

FIG. 4 is a flowchart illustrating paper feeding control according to afirst exemplary embodiment.

FIG. 5 is a flowchart illustrating paper feeding control according to asecond exemplary embodiment.

FIG. 6 is a schematic view of an option cassette in a third exemplaryembodiment.

DESCRIPTION OF THE EMBODIMENTS

Various exemplary embodiments, features, and aspects of the inventionwill be described in detail below with reference to the drawings.

The configuration and the operations of a laser beam printer accordingto a first exemplary embodiment will be described in detail.

In the first exemplary embodiment, it is detected that a conveyanceforce by a conveyance roller for conveying a fed recording medium isreduced, and conveyance assist control is executed to assist theconveyance. When the recording medium is not normally conveyed even ifthe conveyance assist control is executed, it is determined that a paperjam (also referred to as jam) is occurring due to a factor other thanthe reduction in the conveyance force. The details of the operationswill be described below.

FIG. 1 is a schematic view of a laser beam printer 1 according to thepresent exemplary embodiment, illustrating a main body of the laser beamprinter 1 and a paper feeder for feeding paper serving as a recordingmedium.

The main body of the laser beam printer 1 includes a photosensitive drum110 for forming an image. A charging roller 109 for uniformly charging asurface of the photosensitive drum 110, a developing roller 108 fordeveloping an electrostatic latent image formed on the photosensitivedrum 110, and a transfer roller 107 for transferring a developed imageon the recording medium are arranged around the photosensitive drum 110.The main body of the laser beam printer 1 further includes a scannerunit 113 and a mirror 112 as an exposure unit for forming theelectrostatic latent image on the photosensitive drum 110.

As illustrated in FIG. 2, an engine control unit 201 that has receivedprinting information from a printer controller 200 instructs a paperconveyance control unit 203 to convey the recording medium.

First, when a paper feeding solenoid 100 (only a flapper of the solenoidis illustrated in FIG. 1) in a main body cassette 10 serving as aconveyance unit in the main body is pulled, a paper feeding roller 101starts to feed the recording medium. The recording medium is conveyed toa conveyance path 140 via a conveyance roller pair 102.

The recording medium that has been conveyed by the conveyance rollerpair 102 and roller pair 104 is then conveyed to a transfer nip portionincluding the photosensitive drum 110 and the transfer roller 107 afterpassing through a sensor 105 and a paper width sensor 106. When thesensor 105 detects the leading edge of the recording medium, the paperwidth sensor 106 starts to measure the length of the recording medium.When the sensor 105 detects the trailing edge of the recording medium,the paper width sensor 106 finishes measuring the length, to obtain thelength of the recording medium using a period of time elapsed from theleading edge to the trailing edge of the recording medium divided by theconveyance speed of the recording medium.

A paper width sensor (not illustrated) detects the size (the width) in adirection perpendicular to a conveyance direction of the recordingmedium. The size (the length and the width) of the recording medium maybe previously set if it is a standard size (A3, A4, Letter, B5, etc.).

In the transfer nip portion including the photosensitive drum 110 andthe transfer roller 107, the image is transferred onto the recordingmedium.

The recording medium then passes through a fixing device 119. The fixingdevice 119 includes a heater 118 that has been heated, a thermistor 117for detecting the temperature of the heater 118, a fixing film 116 forconducting heat, and a pressure roller 115 for applying pressure to therecording medium. The recording medium passes through a fixing nipportion including the fixing film 116 and the pressure roller 115 sothat the image is fixed on the recording medium. The recording medium onwhich the image has been fixed is conveyed to a conveyance path 141 viaa conveyance roller pair 120.

In the case of one-sided printing, a paper discharge roller 122 conveysthe recording medium and discharges the recording medium out of theprinter 1. In the case of two-sided printing, a two-sided printing motorstarts to rotate in a backward direction after the trailing edge of therecording medium has passed through the conveyance roller pair 120. Therecording medium in the conveyance path 141 is conveyed to a two-sidedconveyance path 142 by the conveyance roller pair 120 that rotates in abackward direction to do printing on its second surface, and a two-sidedconveyance roller pair 131.

The conveyance roller pair 120 includes three conveyance rollers, and isrotatable in a driving direction (forward direction) to discharge therecording medium and in a driving direction (backward direction) toconvey the recording medium to the two-sided conveyance path 142. Thetwo-sided conveyance path 142 includes a regulating plate (notillustrated) that is operated using one side of the conveyance path as abasis. The regulating plate regulates a position in the directionperpendicular to the conveyance direction of the recording medium thatis conveyed to the conveyance path 142.

The recording medium is conveyed to the conveyance path 140 again viathe conveyance roller pair 133. In the transfer nip portion includingthe photosensitive drum 110 and the transfer roller 107, the image istransferred onto the second surface of the recording medium. The fixingdevice 119 fixes the image on the recording medium again, and the paperdischarge roller pair 122 discharges the recording medium out of theprinter 1.

Option cassettes 15 and 16 are removably mounted on the laser beamprinter 1. When the recording medium is fed from the option cassette 15,the recording medium is fed by a paper feeding roller 151, and isconveyed to the conveyance path 140 via a conveyance roller pair 152 andthe conveyance roller pair 102. When the recording medium is fed fromthe option cassette 16, the recording medium is fed by a paper feedingroller 161, and is conveyed to the conveyance path 140 via a conveyanceroller pair 162, the conveyance roller pair 152, and the conveyanceroller pair 102. Operations performed after the recording medium hasbeen conveyed to the conveyance path 140 are similar to theabove-mentioned operations and hence, the description thereof is notrepeated.

FIG. 2 is a block diagram illustrating the configuration of a controlsystem for controlling each of the units in the laser beam printer 1illustrated in FIG. 1. In FIG. 2, the printer controller 200 includes amicrocomputer serving as a control unit, and rasterizes information sentfrom an external apparatus such as a host computer into bit data to doprinting by the printer 1. The printer controller 200 has the functionof reading internal information of the printer 1 in communication anddisplaying the internal information.

The engine control unit 201 includes a microcomputer containing aread-only memory (ROM) and a random access memory (ROM) (notillustrated), sends and receives information to and from the printercontroller 200 by serial communication, and controls the operation ofeach unit in the printer 1 according to an instruction from the printercontroller 200. The engine control unit 201 further sends (reports) theinternal information of the printer 1 to the printer controller 200.

A paper feeding control unit 202 controls the driving of the paperfeeding roller 101 for feeding the recording medium and the rotation andthe stop of the conveyance roller pair 102 according to an instructionfrom the engine control unit 201.

The paper conveyance control unit 203 controls the operations of thephotosensitive drum 110, the transfer roller 107, the pressure roller115, and the paper discharge roller 122 for conveying the recordingmedium that has been fed to reach the conveyance path 140 to a paperdischarge port according to the instruction from the engine control unit201.

A high-voltage control unit 204 controls a high-voltage output duringeach of image forming operations such as charging, development, andtransfer based on the instruction from the engine control unit 201.

An optical system control unit 205 controls the driving and the stop ofa scanner motor (not illustrated) within the scanner unit 113 and thelighting of a laser according to the instruction from the engine controlunit 201.

A sensor input unit 206 sends information from a paper presence/absencesensor (not illustrated), the sensor 105, the paper width sensor 106,and a paper discharge sensor 114 to the engine control unit 201.

A temperature control unit 207 adjusts the temperature of the fixingunit 119 to a temperature designated by the engine control unit 201.

A two-sided conveyance unit control unit 208 controls a two-sidedconveyance unit 130 according to the instruction from the engine controlunit 201.

FIG. 3 is a block diagram illustrating the internal configuration of thepaper feeding control unit 202.

A paper feeding control unit 301 manages and controls the respectivepaper feeding rollers 151 and 161 and the respective conveyance rollerpairs 152 and 162 in the option cassettes 15 and 16, and a paper delaydetection unit 302.

The paper delay detection unit 302 detects whether the fed recordingmedium is detected by respective conveyance path sensors 153 and 163 inthe option cassettes 15 and 16 within a predetermined period of timeelapsed since it was detected by the conveyance path sensors 153 and163.

A paper feeding roller driving unit 303 drives a paper feeding solenoid150 (FIG. 1), to rotate the paper feeding roller 151 one revolution.

A conveyance roller driving unit 304 controls the driving and the stopof the conveyance roller pair 152.

A conveyance path sensor detection unit 305 detects the presence orabsence of the recording medium using the conveyance path sensor 153.

A paper feeding roller driving unit 306 drives a paper feeding solenoid160 (FIG. 1), to rotate the paper feeding roller 161 one revolution.

A conveyance roller driving unit 307 controls the driving and the stopof the conveyance roller pair 162.

A conveyance path sensor detection unit 308 detects the presence orabsence of the recording medium using the conveyance path sensor 163.

The characteristic operations and processing of the laser beam printer 1according to the present exemplary embodiment will be described withreference to FIG. 4. FIG. 4 is a flowchart illustrating control forexecuting an operation for assisting a conveying operation (hereinafterreferred to as a conveyance assist control) when a recording medium isfed from the option cassette 16.

In step 400, if the printer controller 200 instructs the engine controlunit 201 to feed the recording medium from the option cassette 16 andmake a print, the engine control unit 201 starts to feed the recordingmedium from the option cassette 16.

In step 401, the engine control unit 201 also starts to drive therespective conveyance roller pairs 152 and 162 in the option cassettes15 and 16 according to the instruction.

In step 402, the engine control unit 201 drives the paper feeding roller161 in the option cassette 16 to make one revolution so that therecording medium is fed.

In step 403, the engine control unit 201 waits until the recordingmedium reaches the conveyance path sensor 163 in the option cassette 16.If the recording medium has reached the conveyance path sensor 163 inthe option cassette 16 (YES in step 403), then in step 404, the enginecontrol unit 201 starts counting of a timer (not illustrated).

In step 405, the engine control unit 201 waits until the fed recordingmedium reaches the conveyance path sensor 153 in the option cassette 15.

If the recording medium has reached the conveyance path sensor 153 inthe option cassette 15 (YES in step 405), then in step 406, the enginecontrol unit 201 determines that the recording medium has been normallyconveyed, and continues to perform the conveying operation and thesubsequent printing operation. On the other hand, if the recordingmedium has not reached the conveyance path sensor 153 in the optioncassette 15 (NO in step 405), then in step 407, the engine control unit201 determines whether a predetermined period of time has elapsed sincethe timer started counting. If the predetermined period of time haselapsed, the engine control unit 201 determines that the conveyanceroller 162 in the option cassette 16 lacks a conveyance force.

This predetermined period of time is calculated from “((the distancefrom the conveyance path sensor 163 to the conveyance path sensor153)+(margin))÷(conveyance speed)”. The margin is set to 50 mm in thepresent exemplary embodiment. The margin can be set, as needed, based onconveyance variations allowed in the printer 1, for example, theconveyance speed of the recording medium and the configuration of theconveyance path.

If the predetermined period of time has elapsed (YES in step 407), thenin step 408, the engine control unit 201 confirms whether the conveyanceassist control is executed a predetermined number of times.

If the conveyance assist control has been executed the predeterminednumber of times (YES in step 408), then in step 409, the engine controlunit 201 determines that a jam is occurring, and stops the printingoperation. This jam represents defective conveyance in which therecording medium still does not reach the conveyance sensor 153 even ifan assisting operation to control the conveyance assist is executed. Ifthe conveyance assist control has been executed less than thepredetermined number of times (NO in step 408), then in step 410, theengine control unit 201 causes the paper feeding roller 161 in theoption cassette 16 to make one revolution, to assist conveying of therecording medium.

Then, the processing is returned to step 404. In step 404, the enginecontrol unit 201 waits until the recording medium reaches the conveyancepath sensor 153.

In the present exemplary embodiment, if the recording medium does notreach the conveyance path sensor 153 after an elapse of thepredetermined period of time, the conveyance assist control forassisting conveying of the recording medium is executed.

While for each conveyance assist control executed in the presentexemplary embodiment, the paper feeding roller 161 is set to make onerevolution, the number of revolutions of the paper feeding roller 161 isnot limited to one. The number of revolutions can be variably setaccording to the length of the conveyance path and the type of therecording medium. When the conveyance path to the conveyance roller onthe downstream side is long, for example, the number of revolutions maybe set to two. When the type of the recording medium is recycled paper,and the grammage (thickness) thereof is high, the number of revolutionsmay be set to two. In the present exemplary embodiment, the number oftimes the conveyance assist control is executed (the above-mentionedpredetermined number of times) is set to two.

While in the present exemplary embodiment, the recording medium is fedfrom the option cassette 16, the recording medium may also be fed fromthe main body cassette 10 and the option cassette 15. When the recordingmedium is fed from the option cassette 15, it is checked whether therecording medium reaches a conveyance sensor 103. If the recordingmedium has not reached the conveyance sensor 103 even after an elapse ofthe predetermined period of time, the conveyance roller pair 152 mayexecute the conveyance assist control.

As described above, according to the present exemplary embodiment, whenthe conveyance roller 162 lacks the conveyance force, an assistingoperation for assisting the lack of the conveyance force is executed ina low-cost configuration without changing the conventionalconfiguration. This enables a reduction in the occurrence of the jam inthe recording medium. A jam due to a factor other than the lack of theconveyance force can be detected, as in the conventional example.

The configuration and the operations of an image forming apparatusaccording to a second exemplary embodiment will be described in detail.

In the above-mentioned first exemplary embodiment, the conveyance assistcontrol for the recording medium whose conveyance force is insufficienthas been described. The present exemplary embodiment differs from thefirst exemplary embodiment in an operation performed when conveyanceassist control is executed while a plurality of recording media is setin a cassette to do continuous printing.

When the conveyance assist control is executed while the continuousprinting (continuous paper feeding) is performed, if the trailing edgeof the recording medium to be subjected to the conveyance assist controlhas passed through a paper feeding roller, the subsequent recordingmedium may be erroneously fed. In the present exemplary embodiment, anoperation for preventing the subsequent recording medium from beingerroneously fed when the conveyance assist control is executed while thecontinuous paper feeding is performed will be described in detail.

The configuration of the image forming apparatus, the configuration of acontrol block for controlling the image forming apparatus, and theconfiguration of a control block for a paper feeding control unitaccording to the present exemplary embodiment are similar to those inthe first exemplary embodiment and hence, the description thereof is notrepeated.

The present exemplary embodiment will be described with reference toFIG. 5. FIG. 5 is a flowchart illustrating conveyance assist control incontinuously feeding a plurality of recording media from an optioncassette 15 and determination whether the conveyance assist control isexecuted. In FIG. 5, steps in common with those in the first exemplaryembodiment are assigned the same step numbers 400 to 409 and hence, thedescription thereof is not repeated.

If the conveyance assist control is executed less than a predeterminednumber of times (NO in step 408), then in step 510, an engine controlunit 201 determines whether the trailing edge of the recording medium tobe subjected to the conveyance assist control has passed through aconveyance path sensor 163.

If the trailing edge of the recording medium to be subjected to theconveyance assist control has passed through the conveyance path sensor163 (YES in step 510), then in step 409, the conveyance assist controlis not executed because the recording medium to be subjected to theconveyance assist control has passed through a paper feeding roller 161even if the paper feeding roller 161 is driven, and it is determinedthat a jam has occurred. If the trailing edge of the recording medium tobe subjected to the conveyance assist control has not passed through theconveyance path sensor 163 (NO in step 510), then in step 511, theconveyance assist control is executed, to drive a conveyance roller pair162 and the paper feeding roller 161.

In the conveyance assist control in the present exemplary embodiment,assisting of conveyance is controlled by rotating the paper feedingroller 161, similarly to the conveyance assist control described in thefirst exemplary embodiment.

While in the present exemplary embodiment, the recording medium is fedfrom the option cassette 16, the recording medium may also be fed from amain body cassette 10 and an option cassette 15. When the recordingmedium is fed from the option cassette 15, it is checked whether therecording medium reaches a conveyance sensor 103. If the recordingmedium has not reached the conveyance sensor 103 even after an elapse ofa predetermined period of time, a conveyance roller pair 152 may executethe conveyance assist control. By checking whether the trailing edge ofthe recording medium has passed through a paper feeding roller 151, itcan be similarly determined whether the execution of the conveyanceassist control is permitted.

As described in the foregoing, according to the present exemplaryembodiment, when the conveyance assist control is executed while theplurality of recording media is continuously fed, the subsequentrecording medium to be subjected to the conveyance assist control can beprevented from being erroneously fed. The occurrence of a jam in therecording medium due to the lack of a conveyance force can be reduced ina low-cost configuration without changing the conventionalconfiguration, as in the first exemplary embodiment. A jam caused by afactor other than the lack of the conveyance force can be detected, asin the conventional example.

The configuration and the operations of an image forming apparatusaccording to a third exemplary embodiment will be described below indetail.

The conveyance assist control has been described in the first exemplaryembodiment, and the conveyance assist control during the continuouspaper feeding and the determination whether the execution of theconveyance assist control is permitted have been described in the secondexemplary embodiment. The third exemplary embodiment differs from thefirst and second exemplary embodiments in that the number of timesconveyance assist control is executed is variably set.

The configuration of the image forming apparatus according to thepresent exemplary embodiment, the configuration of a control block forcontrolling the image forming apparatus, and the configuration of acontrol block for a paper feeding control unit are similar to those inthe first exemplary embodiment and hence, the description thereof is notrepeated.

The present exemplary embodiment will be described with reference toFIG. 6. FIG. 6 is a schematic view of option cassettes 15 and 16,illustrating the conveyance distance of a recording medium in aconveyance path. FIG. 6 illustrates a distance L1 from the position ofthe leading edge of the recording medium set in a paper feeding tray toa conveyance path sensor 163, a distance L2 from the conveyance pathsensor 163 to a conveyance path sensor 153, a distance Lpap that is thelength of the recording medium set in a cassette (the length in aconveyance direction of the recording medium), and a distance Lpick atwhich a paper feeding roller 161 contacts a surface of the recordingmedium when it makes one revolution.

The conveyance assist control in the present exemplary embodiment issimilar to those in the flowcharts illustrated in FIGS. 5 and 6described in the first and second exemplary embodiments and hence, thedescription thereof is not repeated.

In the present exemplary embodiment, the number of times the conveyanceassist control is executed is variably set based on the length in theconveyance direction of the recording medium and the distance thereof inthe conveyance path.

N is a number of times the conveyance assist control is executed. Thenumber of times N can be set according to the following conditions 1, 2,and 3 relating to the length Lpap in the conveyance direction of therecording medium, the conveyance distances (L1 and L2) of the recordingmedium, and the position of the trailing edge of the recording medium:

Condition 1: “Lpap<L1” (the trailing edge of the recording medium haspassed through the paper feeding roller 161 at timing of the conveyanceassist control)

Condition 2: “L1<Lpap<(L1+L2)” (it is unclear whether the trailing edgeof the recording medium has passed through the paper feeding roller 161at timing of the conveyance assist control)

Condition 3: “(L1+L2)<Lpap” (the trailing edge of the recording mediumhas not passed through the paper feeding roller 161 at timing of theconveyance assist control)

The number of times N the conveyance assist control is executed for eachof the conditions 1 to 3 will be described below.

Under the condition 1, the number of times N the conveyance assistcontrol is executed is set to zero. At the timing of the conveyanceassist control, the trailing edge of the recording medium has alreadypassed through the paper feeding roller 161. When the conveyance assistcontrol is executed, therefore, the subsequent recording medium iserroneously fed.

Under the condition 2, it is unclear whether the trailing edge of therecording medium has passed through the paper feeding roller 161.Therefore, it is determined whether “prevention of a jam by theconveyance assist control” or “erroneous feeding of the subsequentrecording medium” should be selected. The number of times N theconveyance assist control is executed is decided based on thedetermination. For example, when a recording medium of a type having badconveyance properties has passed or when the number of copies to beprinted is increased so that it is likely that dirt adheres to aconveyance roller, “prevention of a jam by the conveyance assistcontrol” is given priority, and the number of times N is set to one. Inthe other cases, “erroneous feeding of the subsequent recording medium”is given priority, and the number of times N is set to zero.

Under the condition 3, the trailing edge of the recording medium has notpassed through the paper feeding roller 161, and the number of times Nthe conveyance assist control is executed is determined by the followingequation:

N=(Lpap−(L1+L2)/Lpick+1  (1)

Although in the present exemplary embodiment, the recording medium isfed from the option cassette 16, the number of times N the conveyanceassist control is executed may be set by the length of the recordingmedium and the distance thereof in the conveyance path when therecording medium is fed from each of the main body cassette 10 and theoption cassette 15.

As described above, according to the present exemplary embodiment, theoptimum number of times the conveyance assist control is executed can bedetermined based on the length in the conveyance direction of therecording medium and the conveyance distance thereof in the conveyancepath. The occurrence of a jam in the recording medium due to the lack ofa conveyance force can be reduced in a low-cost configuration withoutchanging the conventional configuration, as in the first exemplaryembodiment. A jam caused by a factor other than the lack of theconveyance force can be detected, as in the conventional example. Whenthe conveyance assist control is executed while a plurality of recordingmedia is continuously fed, the subsequent recording medium to besubjected to the conveyance assist control can be prevented from beingerroneously fed, as in the second exemplary embodiment.

The other exemplary embodiments will be then described.

In addition to the control described in the first to third exemplaryembodiments, it is also possible to switch the number of timesconveyance assist control is executed in consideration of the type ofpaper and the degree of wear of a paper feeding roller, for example.

For example, it is possible to switch the number of times the conveyanceassist control is executed between when the paper is plain paper andwhen it is thick paper having a higher grammage (weight per unit area)than the plain paper. As an example, the number of times the conveyanceassist control is executed can be set to two for the plain paper, whilethe number of times is set to three for the thick paper. A moreconveyance force is applied for the thick paper than for the plainpaper. When the thick paper and the plain paper are conveyed in asimilar way, the conveyance properties of the thick paper tend to beworse than those of the plain paper. Therefore, the number of times theconveyance assist control is executed is set larger for the thick paperthan for the plain paper. It is also effective to predict the degree ofwear of the paper feeding roller from the number of copies to print andgradually increase the number of times the conveyance assist control isexecuted as the cumulative number of copies to print increases. Forexample, by counting the cumulative number of copies to be printed, thenumber of times the conveyance assist control is executed can begradually switched to two when it is 0 to 5000, three when 5001 to10000, and four when it is 10001 or more.

The number of times the conveyance assist control is executed can bemore appropriately set and switched when thus the number of times isswitched in consideration of the type of the paper and the degree ofwear of the paper feeding roller.

While in the plurality of exemplary embodiments described above, thepaper feeding roller is in a D-cut shape, the conveyance assist controlis similarly applicable to a configuration in which paper feedingrollers abut on and separate from each other to perform feeding. In thiscase, when it is determined that a conveyance roller lacks a conveyanceforce, for example, conveyance assist control can be executed whichdrives paper feeding rollers for a predetermined period of time whileabutting on each other.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all modifications, equivalent structures, and functions.

This application claims priority from Japanese Patent Application No.2009-076768 filed Mar. 26, 2009, which is hereby incorporated byreference herein in its entirety

1. An paper feeder comprising: a feeding unit configured to execute anoperation for feeding a recording medium and then, stop the operation; aconveyance unit configured to convey the fed recording medium fed by thepaper feeding unit; and a detection unit configured to detect theconveyed recording medium conveyed by the conveyance unit, wherein whenthe detection unit does not detect the conveyed recording medium at apredetermined timing, the feeding unit executes an assisting operationfor assisting conveying of the recording medium.
 2. The paper feederaccording to claim 1, wherein when the detection unit does not detectthe recording medium even if the assisting operation is executed, it isdetermined that defective conveyance of the recording medium hasoccurred and a conveying operation is stopped.
 3. The paper feederaccording to claim 1, wherein when the assisting operation is executedwhile the feeding unit feeds a plurality of recording media, it isdetermined whether the execution of the assisting operation is permitteddepending on whether a trailing edge of the recording medium has passedthrough the feeding unit.
 4. The paper feeder according to claim 1,wherein a number of times the assisting operation is executed is setbased on a length in a conveyance direction of the recording medium anda distance in a conveyance path from a leading edge of the recordingmedium that has not been fed, to the detection unit.
 5. The paper feederaccording to claim 1, wherein a number of times the assisting operationis executed is set based on a type of the recording medium.
 6. The paperfeeder according to claim 1, wherein a number of times the assistingoperation is executed is set based on a degree of wear of the feedingunit.
 7. An paper feeder apparatus comprising: a feeding unit configuredto execute an operation for feeding a recording medium and then, stopthe operation; a conveyance unit configured to convey the fed recordingmedium fed by the paper feeding unit; a detection unit configured todetect the conveyed recording medium conveyed by the conveyance unit;and an image forming unit configured to form an image on the detectedrecording medium, wherein when the detection unit does not detect theconveyed recording medium at a predetermined timing, the feeding unitexecutes an assisting operation for assisting conveying of the recordingmedium.
 8. The paper feeder apparatus according to claim 7, wherein whenthe detection unit does not detect the recording medium even if theassisting operation is executed, it is determined that defectiveconveyance of the recording medium has occurred and a conveyingoperation is stopped.
 9. The paper feeder apparatus according to claim7, wherein when the assisting operation is executed while the feedingunit feeds a plurality of recording media, it is determined whether theexecution of the assisting operation is permitted depending on whether atrailing edge of the recording medium has passed through the feedingunit.
 10. The paper feeder apparatus according to claim 7, wherein anumber of times the assisting operation is executed is set based on alength in a conveyance direction of the recording medium and a distancein a conveyance path from a leading edge of the recording medium thathas not been fed, to the detection unit.
 11. The paper feeder apparatusaccording to claim 7, wherein a number of times the assisting operationis executed is set based on a type of the recording medium.
 12. Thepaper feeder apparatus according to claim 7, wherein a number of timesthe assisting operation is executed is set based on a degree of wear ofthe feeding unit.